Data storage device compensating for head/tape wear

ABSTRACT

A data storage device is disclosed comprising at least one head configured to access a magnetic tape. A plurality of access commands are stored in a command queue, and a wear value is generated for each access command in the command queue, wherein the wear value represents a level of wear on the head or magnetic tape associated with executing the access command. An execution order for the access commands is generated based on the wear values, and at least one of the access commands is executed based on the execution order.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 63/069,870, filed on Aug. 25, 2020, which is herebyincorporated by reference in its entirety.

BACKGROUND

Conventional tape drive storage systems comprise a magnetic tape woundaround a dual reel (reel-to-reel cartridge) or a single reel (endlesstape cartridge), wherein the reel(s) are rotated in order to move themagnetic tape over one or more transducer heads during write/readoperations. The format of the magnetic tape may be single track ormultiple tracks that are defined linearly, diagonally, or arcuate withrespect to the longitudinal dimension along the length of the tape. Witha linear track format, the heads may remain stationary relative to thelongitudinal dimension of the tape, but may be actuated in a lateraldimension across the width of the tape as the tape moves past the heads.With a diagonal or arcuate track format, the heads may be mounted on arotating drum such that during access operations both the heads and tapeare moved relative to one another (typically in opposite directionsalong the longitudinal dimension of the tape).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a data storage device according to an embodimentcomprising at least one head configured to access a magnetic tape.

FIG. 1B is a flow diagram according to an embodiment wherein queuedaccess commands are executed in an order that reduces head/tape wear.

FIG. 1C shows a data storage device comprising a cartridge assemblycomprising a magnetic tape, and a tape drive assembly configured toaccess the magnetic tape.

FIG. 2A shows an example of a number of files written to data tracks ofthe magnetic tape according to an embodiment.

FIG. 2B shows an example database maintained for the files stored on themagnetic tape which facilitates generating the execution order accordingto an embodiment.

FIG. 3A shows different examples of access commands queued in a commandqueue and the execution order generated to reduce head/tape wear.

FIG. 3B shows how the execution order is generated for each example ofFIG. 3A according to an embodiment.

FIG. 4 shows an embodiment wherein the wear values of the accesscommands are weighted based on an overall wear measurement for the datastorage device.

FIG. 5A shows an embodiment wherein an accumulated wear measurement ismaintained for each of a plurality of segments of the magnetic tape.

FIG. 5B shows an embodiment wherein each segment of the magnetic tape inFIG. 5A consists of an error correction code (ECC) codeword.

FIGS. 6A-6C show an embodiment wherein part of the magnetic tape isreserved for longitudinal scanning the tape and laterally seeking thehead.

DETAILED DESCRIPTION

FIGS. 1A and 1B show a data storage device according to an embodimentcomprising at least one head 2 configured to access a magnetic tape 4.The data storage device further comprises control circuitry 6 configuredto execute the flow diagram of FIG. 1B, wherein a plurality of accesscommands are stored in a command queue (block 8) and a wear value isgenerated for each access command in the command queue (block 10),wherein the wear value represents a level of wear on the head ormagnetic tape associated with executing the access command. An executionorder is generated for the access commands based on the wear values(block 12), and at least one of the access commands is executed based onthe execution order (block 14).

In the embodiment of FIG. 1A, the data storage device comprises anembedded magnetic tape 4 installed into a tape drive assembly which, inone embodiment, may be the same form factor as a conventional diskdrive. In another embodiment shown in FIG. 1C, the magnetic tape 4 maybe housed in a cartridge assembly 3 that is inserted into (and ejectedfrom) a tape drive assembly 5 similar to a conventional tape drivemanufactured under the Linear Tape-Open (LTO) standard. In oneembodiment, the tape drive assembly 5 comprises the head 2 configured toaccess the magnetic tape 4, and the control circuitry 6 configured toexecute the flow diagram of FIG. 1B.

In one embodiment, there is head/tape wear as the magnetic tape movespast the head (or as the head moves relative to the magnetic tape). Thehead/tape wear may be exacerbated by seeking the head laterally acrossthe magnetic tape and/or by reversing the scan direction of the head.Since the accumulative wear on the head or the magnetic tape mayeventually render at least part of the magnetic tape unusable, in oneembodiment the access commands received from a host are processed so asto reduce the head/tape wear, thereby extending the life of the datastorage device. In one embodiment, the execution order of the accesscommands is configured based on a wear value generated for each accesscommand. In another embodiment, a segment wear map is maintained for themagnetic tape so as to spread out the tape wear when executing accessoperations.

FIG. 2A shows an embodiment wherein the magnetic tape is formatted tocomprise two bands of data tracks (Band 0 and Band 1), where each bandconsists of three data tracks (data tracks 0-5). When accessing Band 0the magnetic tape scans in a forward direction from right to left, andwhen accessing Band 1 the magnetic tape scans in a reverse directionfrom left to right. The magnetic tape is divided into six segments (0-5)from the beginning of tape (BOT) to the end of tape (EOT). In thisexample, there are 20 files stored in the six data tracks where eachfile may include one or more of the segments. For example, File 1 isstored in Segment 0 of data track 2, whereas File 16 starts at Segment 0of data track 0 and ends at Segment 4 of data track 5. Also in thisexample, the files are written from the BOT to the EOT in a forwarddirection, and then from the EOT to the BOT in the reverse direction.For example, after writing Segment 5 of data track 2 for File 5 in theforward direction, the head seeks to data track 3 and begins writingFile 6 in the reverse direction. FIG. 2B shows a table maintained by thecontrol circuitry 6 representing the physical location of each file onthe magnetic tape (for the example of FIG. 2A) which facilitatesgenerating a wear value for each access command in a command queue.

FIG. 3A shows an example of an active access command and three pendingaccess commands stored in a command queue. In this example, each accesscommand corresponds to a file number from the example shown in FIGS. 2Aand 2B. That is, in this example each access command corresponds to anaccess (write or read) of the entire file. Each example shows the activeand queued access commands, together with the execution order generatedthat minimizes the head/tape wear. The examples of FIG. 3A areprogressive in that each example corresponds to a new access commandbeing received from the host, and the corresponding new execution ordergenerated for the queued access commands.

FIG. 3B shows an analysis of each example in FIG. 3A, wherein a wearvalue is assigned to each type of movement needed to access a nextaccess command, including a lateral value (lateral seek length), whethernot a reversal in direction is needed, and the scan length in segmentsbefore the head reaches the first segment of the next access command. Inthis embodiment, the wear value is incremented for each data track inthe seek length and incremented for each segment in the scan length (theweight value for a reversal is one). Referring to the analysis ofexample 1, if File 13 is accessed after File 1 the head will seek onedata track from Track 2 to Track 1 (see FIG. 2A), and the scan lengthwill be four segments (1-4). There is no reversal in the scan direction,and so the Reversal value is zero resulting in an overall wear value of5. The overall wear value associated with selecting File 4 as the nextaccess command is 2, which is the “winner” since it is the lowest wearvalue. As shown in FIG. 3B, a wear value is then generated for the nextaccess command after File 4 (i.e., for Files 13 and 17), wherein File 13is the winner resulting in an execution order of {1, 4, 13, 17} as shownin FIG. 3A. In example 2 of FIG. 3A, the next access command receivedfrom the host is to access File 10 which is inserted into the commandqueue. The execution order is then generated based on the analysis shownin FIG. 3B which results in the updated execution order of {4, 13, 17,10} and so on for example 3 and example 4.

Any suitable weighting may be assigned to each type of movement neededto access a next access command (i.e., other than the weighting shown inthe example of FIG. 3B). For example, in one embodiment the wear valuemay be incremented more than once for each data track in the seek lengthif seeking the head laterally across the magnetic tape results in morewear than scanning the magnetic tape longitudinally across the head.Similarly, the wear value may be incremented more than once for areversal of the scan direction when a reversal results in more wear (ofthe head or tape) than scanning the magnetic tape across the head.

In one embodiment, an access latency may be generated for each accesscommand in the command queue, wherein the access latency includes anaccess time for the head to reach a segment of the magnetic tape inorder to execute the access command. For example, there is an accesslatency associated with the scan length and the scan speed that themagnetic tape moves across the head, as well as an access latencyassociated with the seek length and the seek speed of the head laterallyacross the magnetic tape. Similarly, there is an access latencyassociated with reversing the direction of the magnetic tape. In oneembodiment, the execution order of the queued access commands may begenerated based on a wear value as well as an access latency associatedwith each access command. For example, in one embodiment the executionorder may be generated so as to minimize a predetermined weightedcombination of the wear values and the access latencies. When longevityis considered more important than performance, the weighting may beskewed toward minimizing the wear value, and when performance isconsidered more important, the weighting may be skewed toward minimizingthe access latency.

FIG. 4 shows an embodiment wherein the control circuitry 6 is configuredto maintain an overall wear measurement representing an overallhead/tape wear over the life of the data storage device. In oneembodiment, as the overall wear measurement increases due to accessingthe magnetic tape over time, the weighting of the wear values increasesso that the execution order is generated in a manner that is skewedtoward reducing the head/tape wear rather than reducing the accesslatency. Accordingly in this embodiment, the data storage device mayexhibit better performance in the beginning with a predetermineddegradation in performance as the overall wear measurement increases. Inthis manner, the data storage devices that are lightly used, or usedwith minimal access scans/seeks, may maintain a higher level ofperformance for a longer period of time.

FIG. 5A shows a format of the magnetic tape according to an embodimentcomprising a plurality of data tracks, wherein each data track comprisesa plurality of segments. In this embodiment, the control circuitry 6 isconfigured to maintain an accumulated wear measurement for each segment,and seek the head laterally across the magnetic tape based on theaccumulated wear measurements. In one embodiment, the accumulated wearmeasurement for each segment may be updated at a different ratedepending on the wear movement. For example, the wear measurement may beincremented by a first amount when the magnetic tape scans past thesegment, by a second amount when the head seeks through the segment, andby a third amount when the head reverses direction while over thesegment.

In the example of FIG. 5A, the shade of each segment represents theaccumulated wear measurement where a lighter shade represents less wearof the segment and a darker shade represents more wear. During a seekoperation, the control circuitry 6 may control the position of the headso as to spread out the wear on the segments of the magnetic tape. Forexample, after the head finishes accessing segment 16 and then seeks tosegment 18, the control circuitry 6 avoids seeking through segment 20since it has a high accumulated wear measurement. Instead, the controlcircuitry 6 scans the head through segment 22 and then step seeks fromdata track 2 to data track 3 at segments 24 and 26, and then step seeksto data track 3 prior to reaching the target segment 18. That is, thecontrol circuitry 6 step seeks the head across the data tracks in anorder that spreads the resulting tape wear over the segments having thelowest accumulated wear measurement, thereby spreading the tape wearevenly over the segments. In another embodiment, the control circuitry 6may serpentine seek the head across the data tracks in order to seekthrough as well as scan through the segments having the lowestaccumulated wear measurement.

In another example shown in FIG. 5A, after the head finishes accessingsegment 28 and then seeks to segment 30, the control circuitry 6performs the direction reversal at segment 26 rather than at segment 20which has a high accumulated wear measurement. That is, in thisembodiment scanning through segment 20 results in less wear thanperforming a direction reversal at the segment, and therefore thereversal direction is deferred until segment 26 which has a loweraccumulated wear measurement.

In one embodiment, the accumulated wear measurement for each segment maybe taken into account when generating the execution order of the queuedaccess commands as described above. That is, the wear value generatedfor each pending access command may be based on the head/tape wear dueto the movement of the head relative to the tape, as well as theaccumulated wear measurement for each of the segments through thecandidate seek trajectories.

FIG. 5B shows an embodiment wherein each segment of the tape such asshown in FIG. 5A consists of an error correction code (ECC) codeword,where the accumulated wear of each segment affects the ability of theECC codeword to recover the segment during a read operation. Each ECCcodeword may cover any suitable length of the magnetic tape, such as alow density parity check (LDPC) codeword that covers a single datasector, or a parity sector codeword that covers multiple data sectors(multiple LDPC codewords). In yet another embodiment, the ECC codewordmay cover a super block of parity sector codewords, for example, anerasure codeword capable of recovering multiple erased LDPC codewords.

In one embodiment, the segments shown in FIG. 5A may be refreshedperiodically in order to compensate for degradation over time andtemperature, degradation due to intertrack interference, etc. In oneembodiment, the refresh threshold that triggers a refresh operation of asegment may be adjusted inversely with the accumulated wear measurementof the segment. That is as the accumulated wear measurement of a segmentincreases, the refresh of the segment may be triggered sooner tocompensate for the wear degradation of the magnetic tape.

In one embodiment, each segment of the magnetic tape such as shown inFIG. 5A may comprise a plurality of data tracks that are accessedconcurrently using a plurality of heads integrated into a head bar. Inthis embodiment, the control circuitry 6 may seek the head bar to atarget band of data tracks corresponding to a target segment. Also inthis embodiment, the ECC codeword that forms a segment in FIG. 5A mayconsist of multiple data track codewords (e.g., LDPC codewords) that areconcurrently written to the segment using the head bar.

FIG. 6A shows an embodiment wherein at least one longitudinal reservedband 32 is defined on the magnetic tape, wherein data is not recorded inthe longitudinal reserved band 32. When executing an access command toaccess a longitudinal data band (e.g., data track or tracks), the headis scanned along the longitudinal reserved band 32 in order to reach thelongitudinal location of the data band. A lateral seek then positionsthe head at the beginning of the data band and the head accesses thedata band while the head scans along a length of the data band. In thisembodiment, the head may scan along a longitudinal band of the magnetictape (e.g., reserved band or data band) by actuating the tape past thehead or by actuating the head past the tape. Since in this embodimentdata is not recorded in the longitudinal reserved band 32, the tape weardue to scanning the head during access operations does not adverselyimpact previously recorded data, or degrade the recording integrity ofthe magnetic tape.

FIG. 6B shows an embodiment wherein at least one lateral reserved band341 is defined on the magnetic tape, wherein data is not recorded in thelateral reserved band 341. When executing an access command to access alongitudinal data band (e.g., data track or tracks), the head seeksalong the lateral reserved band 341 in order to reach the laterallocation of the data band. The head then accesses the data band whilethe head scans along a length of the data band. In this embodiment, thehead may scan along a a data band by actuating the tape past the head orby actuating the head past the tape. Since in this embodiment data isnot recorded in the lateral reserved band 341, the tape wear due toseeking the head laterally during access operations does not adverselyimpact previously recorded data, or degrade the recording integrity ofthe magnetic tape.

FIG. 6C shows an embodiment wherein at least one longitudinal reservedband 32 and at least one lateral reserved band 341 are defined on themagnetic tape (i.e., FIG. 6C combines the embodiments of FIGS. 6A and6B). This embodiment avoids tape wear in the data areas of the disk bothwhen scanning the head longitudinally along a longitudinal reserved bandas well as seeking the head laterally along a lateral reserved band inorder to execute access commands.

In one embodiment, the wear values for generating the execution order ofqueued access commands as described above may be generated relative to areserved area of the magnetic tape, such as the longitudinal reservedband of FIG. 6A and/or the lateral reserved band of FIG. 6B. That is,the possible scan/seek trajectories for each queued access command andcorresponding wear values may be generated relative to the optional useof the reserved areas during the scan/seek operations. In oneembodiment, the optimal scan/seek trajectory that minimizes weightedfactors (such as tape wear and access latency) may avoid using thereserved area(s) of the magnetic tape.

In one embodiment, the data storage device may be configured by the hostto achieve a desired level of performance versus longevity. For example,a particular customer may configure the data storage device to reducethe significance of head/tape wear on command ordering or seek latencyin order to achieve a higher level of performance. The data storagedevice may reduce the significance of head/tape wear by reducing theweighting of the wear values used for command ordering, or by executingseek operations more aggressively with less consideration given tospreading out tape wear as described above. In this embodiment, tradinglongevity for performance may lead to an increased “swap out” frequencydue to degraded or failing data storage devices that eventually sufferfrom excessive head/tape wear.

Any suitable control circuitry may be employed to implement the flowdiagrams in the above embodiments, such as any suitable integratedcircuit or circuits. For example, the control circuitry may beimplemented within a read channel integrated circuit, or in a componentseparate from the read channel, such as a data storage controller, orcertain operations described above may be performed by a read channeland others by a data storage controller. In one embodiment, the readchannel and data storage controller are implemented as separateintegrated circuits, and in an alternative embodiment they arefabricated into a single integrated circuit or system on a chip (SOC).In addition, the control circuitry may include a suitable preamp circuitimplemented as a separate integrated circuit, integrated into the readchannel or data storage controller circuit, or integrated into a SOC.

In one embodiment, the control circuitry comprises a microprocessorexecuting instructions, the instructions being operable to cause themicroprocessor to perform the flow diagrams described herein. Theinstructions may be stored in any computer-readable medium. In oneembodiment, they may be stored on a non-volatile semiconductor memoryexternal to the microprocessor, or integrated with the microprocessor ina SOC. In yet another embodiment, the control circuitry comprisessuitable logic circuitry, such as state machine circuitry. In someembodiments, at least some of the flow diagram blocks may be implementedusing analog circuitry (e.g., analog comparators, timers, etc.), and inother embodiments at least some of the blocks may be implemented usingdigital circuitry or a combination of analog/digital circuitry.

In addition, any suitable electronic device, such as computing devices,data server devices, media content storage devices, etc. may comprisethe storage media and/or control circuitry as described above.

The various features and processes described above may be usedindependently of one another, or may be combined in various ways. Allpossible combinations and subcombinations are intended to fall withinthe scope of this disclosure. In addition, certain method, event orprocess blocks may be omitted in some implementations. The methods andprocesses described herein are also not limited to any particularsequence, and the blocks or states relating thereto can be performed inother sequences that are appropriate. For example, described tasks orevents may be performed in an order other than that specificallydisclosed, or multiple may be combined in a single block or state. Theexample tasks or events may be performed in serial, in parallel, or insome other manner. Tasks or events may be added to or removed from thedisclosed example embodiments. The example systems and componentsdescribed herein may be configured differently than described. Forexample, elements may be added to, removed from, or rearranged comparedto the disclosed example embodiments.

While certain example embodiments have been described, these embodimentshave been presented by way of example only, and are not intended tolimit the scope of the inventions disclosed herein. Thus, nothing in theforegoing description is intended to imply that any particular feature,characteristic, step, module, or block is necessary or indispensable.Indeed, the novel methods and systems described herein may be embodiedin a variety of other forms; furthermore, various omissions,substitutions and changes in the form of the methods and systemsdescribed herein may be made without departing from the spirit of theembodiments disclosed herein.

What is claimed is:
 1. A data storage device configured to access amagnetic tape, the data storage device comprising: at least one headconfigured to access the magnetic tape; and control circuitry configuredto: store a plurality of access commands in a command queue; generate awear value for each access command in the command queue, wherein thewear value represents a level of wear on the head or magnetic tapeassociated with executing the access command; generate an executionorder for the access commands based on the wear values; and execute atleast one of the access commands based on the execution order.
 2. Thedata storage device as recited in claim 1, wherein the data storagedevice comprises the magnetic tape.
 3. The data storage device asrecited in claim 1, wherein: the magnetic tape is housed in a cartridgeassembly; and the data storage device comprises a tape drive assemblyconfigured to receive the cartridge assembly.
 4. The data storage deviceas recited in claim 1, wherein the wear value comprises a longitudinaldistance of the magnetic tape for the head to reach a segment of themagnetic tape in order to execute the access command.
 5. The datastorage device as recited in claim 1, wherein the wear value comprises alateral distance of the magnetic tape for the head to seek to a datatrack of the magnetic tape in order to execute the access command. 6.The data storage device as recited in claim 1, wherein the wear valuecomprises a turnaround direction of the magnetic tape in order toexecute the access command.
 7. The data storage device as recited inclaim 1, wherein the wear value is generated by weighting a plurality offactors including at least: a longitudinal distance of the magnetic tapefor the head to reach a segment of the magnetic tape in order to executethe access command; a lateral distance of the magnetic tape for the headto seek to a data track of the magnetic tape in order to execute theaccess command; and a turnaround direction of the magnetic tape in orderto execute the access command.
 8. The data storage device as recited inclaim 1, wherein the control circuitry is further configured to:generate an access latency for each access command in the command queue,wherein the access latency includes an access time for the head to reacha segment of the magnetic tape in order to execute the access command;and generate the execution order based on the wear values and the accesslatencies.
 9. The data storage device as recited in claim 8, wherein thecontrol circuitry is further configured to: maintain an overall wearmeasurement; weight the wear values based on the overall wearmeasurement; and generate the execution order based on the weighted wearvalues and the access latencies.
 10. The data storage device as recitedin claim 1, wherein the control circuitry is further configured to:maintain an accumulated wear measurement for each of a plurality ofsegments of the magnetic tape; weight the wear values based on theaccumulated wear measurements; and generate the execution order based onthe weighted wear values.
 11. The data storage device as recited inclaim 10, wherein the control circuitry is further configured togenerate the execution order so as to spread out the wear of themagnetic tape across the plurality of segments.
 12. The data storagedevice as recited in claim 10, wherein each segment consists of an errorcorrection code (ECC) codeword.
 13. A data storage device configured toaccess a magnetic tape, the data storage device comprising: at least onehead configured to access the magnetic tape; and control circuitryconfigured to: maintain an accumulated wear measurement for each of aplurality of segments of the magnetic tape; and seek the head laterallyover the magnetic tape based on the accumulated wear measurements. 14.The data storage device as recited in claim 13, wherein the data storagedevice comprises the magnetic tape.
 15. The data storage device asrecited in claim 13, wherein: the magnetic tape is housed in a cartridgeassembly; and the data storage device comprises a tape drive assemblyconfigured to receive the cartridge assembly.
 16. The data storagedevice as recited in claim 13, wherein the control circuitry is furtherconfigured to seek the head laterally over the magnetic tape in order tospread out wear of the magnetic tape across the plurality of segments.17. The data storage device as recited in claim 16, wherein each segmentconsists of an error correction code (ECC) codeword.
 18. The datastorage device as recited in claim 13, wherein the control circuitry isfurther configured to delay the seek in order to prevent the head frompassing over one of the segments based on the accumulated wearmeasurement for the segment.
 19. The data storage device as recited inclaim 13, wherein the control circuitry is further configured to stepseek the head laterally over the magnetic tape based on the accumulatedwear measurements.
 20. The data storage device as recited in claim 13,wherein the control circuitry is further configured to serpentine seekthe head laterally over the magnetic tape based on the accumulated wearmeasurements.
 21. The data storage device as recited in claim 13,wherein the control circuitry is further configured to reverse adirection of the head prior to seeking the head laterally over themagnetic tape based on the accumulated wear measurements.
 22. A datastorage device configured to access a magnetic tape, the data storagedevice comprising: at least one head configured to access the magnetictape; and a means for generating a wear value for each of a plurality ofaccess command in a command queue, wherein the wear value represents alevel of wear on the magnetic tape associated with executing the accesscommand; and a means for executing the access commands based on anexecution order determined based on the wear values.
 23. The datastorage device as recited in claim 22, further comprising a means formaintaining an accumulated wear measurement for each of a plurality ofsegments of the magnetic tape.
 24. The data storage device as recited inclaim 23, wherein the execution order is determined based on the wearvalues and the accumulated wear measurements.
 25. A data storage deviceconfigured to access a magnetic tape, the data storage devicecomprising: at least one head configured to access the magnetic tape;and control circuitry configured to: define at least one longitudinalreserved band on the magnetic tape, wherein data is not recorded in thelongitudinal reserved band; receive an access command to access alongitudinal data band on the magnetic tape; and scan the head along thelongitudinal reserved band and then scan the head along the longitudinaldata band in order to execute the access command.
 26. A data storagedevice configured to access a magnetic tape, the data storage devicecomprising: at least one head configured to access the magnetic tape;and control circuitry configured to: define at least one lateralreserved band on the magnetic tape, wherein data is not recorded in thelateral reserved band; receive an access command to access alongitudinal data band on the magnetic tape; and seek the head along thelateral reserved band and then scan the head along the longitudinal databand in order to execute the access command.